Various industrial production processes of acetic acid have been known. Among others, an industrially excellent process includes a process which comprises continuously allowing methanol to react with carbon monoxide with the use of a metal catalyst (such as a rhodium catalyst), an ionic iodide (e.g., lithium iodide), and methyl iodide in the presence of water to give acetic acid. Moreover, recently improvement in reaction conditions and catalysts was investigated, and an industrial process for producing acetic acid with a highly efficient production has been developed by addition of a catalyst stabilizer (such as an iodide salt) and the reaction under a low water content condition compared with the conventional condition.
According to the process, usually, acetic acid is produced by allowing methanol to react with carbon monoxide, subjecting the resulting reaction mixture containing acetic acid to distillation (flash distillation) in a flasher (flash evaporator), subjecting a component vaporized by the distillation to further distillation, and separating (further purifying) a component containing acetic acid. The reaction mixture contains product acetic acid, methyl iodide, and others. When the concentration of hydrogen iodide in the flash evaporator is increased in response to the distillation conditions (e.g., a composition (formulation) change, a pressure change, and a temperature change), the corrosion of the flash evaporator may be precipitated. That is, since the flash distillation is a continuous single distillation, usually hydrogen iodide is not condensed in the flash evaporator (or the effect of condensation hardly reveals). However, in addition to hydrogen iodide contained in the reaction mixture, there is hydrogen iodide newly produced in the flash evaporator (for example, hydrogen iodide is produced by a reaction of methyl iodide with acetic acid due to a lowering of the concentration of methyl acetate or produced by a reaction of lithium iodide with acetic acid due to an increase in lithium iodide). When a component containing the hydrogen iodide is subjected to a distillation column for acetic acid separation or when a residue (liquid residue or bottom fraction) after separation of the vaporized component is recycled to the reactor, the reaction system may be adversely affected, and additionally the corrosion of peripheral device(s) may be precipitated. Therefore, it is preferable that rising of the concentration of hydrogen iodide in the flash evaporator be prevented. Although a technique for inhibiting condensation of hydrogen iodide in a distillation column such as plate column, packed column has been already known, a technique closely focused on hydrogen iodide in a flash distillation for the reaction mixture is not known.
For example, Japanese Patent Application Laid-Open No. 2006-160645 (JP-2006-160645A, Patent Document 1) discloses a process for distilling a mixture containing hydrogen iodide and water, which comprises distilling the mixture having a water content of not more than 5% by weight in a distillation system to prevent condensation of hydrogen iodide in the distillation system. With respect to a mixture applying the process, the document discloses that the process can be applied to a reaction mixture (or liquid reaction composition) or to a light component which is separated from the reaction mixture by a first distillation and is rich in a low boiling point component (e.g., water, an alcohol, an alkyl iodide, a carboxylic acid or an acid anhydride thereof, a carboxylate ester, and hydrogen iodide). In Examples of this document, a process solution (specifically, a volatile component separated by a flash distillation of a reaction mixture) free from an ionic iodide (such as lithium iodide) is examined for the effect of the water concentration on the hydrogen iodide condensation. As described above, the purpose of Patent Document 1 is to condense hydrogen iodide in distillation, and the decrease of hydrogen iodide in a flash evaporator has not been examined.
Japanese Patent Application Laid-Open No. 6-40998 (JP-6-40998A, Patent Document 2) discloses a process for producing acetic acid by allowing methanol to react with carbon monoxide in the presence of a rhodium catalyst and methyl iodide, the process comprising a step for continuously conducting the reaction at a water concentration of not more than 10 wt % in the reaction solution, continuously withdrawing the reaction solution to introduce an evaporation step in which the pressure is lower than the reaction condition, and separating a vaporizing component and a non-vaporizing component containing rhodium; and in the step, (a) hydrogen is introduced and the evaporation is conducted in the condition that the hydrogen partial pressure is at least not less than 0.1 atmospheres, or/and (b) the non-vaporizing component containing the separated rhodium is treated with at least hydrogen having a hydrogen partial pressure of not less than 0.1 atmospheres and not less than 0.1 atmospheres of carbon monoxide and then returned to the reactor for recycling. The document discloses in Examples that the reaction solution is subjected to flash distillation at a bottom liquid temperature of 120 to 138° C. to evaporate to half that volume, a condensed solution containing 0.5% by weight of methyl iodide, 0.2% by weight of methyl acetate, and others is obtained. However, no consideration is taken in hydrogen iodide in the flash distillation.